Acetic anhydride is a well-known raw material widely used in the chemical industry, which is mainly used for producing chemicals such as cellulose acetate and is an important raw material for synthesizing medicines, flavors, dyes, etc. There are currently three industrial processes for producing acetic anhydride, including the ketone process, the acetaldehyde oxidation process and the methyl acetate carbonylation process.
The ketone process is carried out by dissociating one water molecule or methane from the raw material, acetic acid or acetone, at a high temperature to form ketone, which then reacts with acetic acid to form acetic anhydride. This process is carried out at a reaction temperature of up to 750° C. and thus will gradually go out of use in the future due to its high energy-consuming demand. The acetaldehyde oxidation process is carried out by oxidizing acetaldehyde into peracetic acid, in the presence of the metal catalyst such as manganese, cobalt, nickel, copper, etc., which further reacts with acetaldehyde to form acetic anhydride and the by-product, water. Acetic anhydride will further be hydrolyzed into acetic acid ant the yield of acetic anhydride will thus be reduced. Therefore, the product thereof is the mixture of acetic anhydride and acetic acid. The methyl acetate carbonylation process for producing acetic anhydride uses methyl acetate and carbon monoxide as the raw materials to produce acetic anhydride in the presence of transition metal catalysts and iodide promoter. Currently, the old-fashioned ketone process, which is small in scale and is adopted by many manufacturers, is predominant; however, for commercially producing acetic anhydride in large scale, the methyl acetate carbonylation process is used due to the high energy consuming and other drawbacks of the ketone process.
The methyl acetate carbonylation process for producing acetic anhydride is an expanded application of the methanol carbonylation process for producing acetic acid. The difference between the methyl acetate carbonylation process and the methanol carbonylation process is the water content of the reaction solution; the reaction solution of the former has to be kept in anhydrous conditions, while the reaction solution of the latter can have any water ratio of between 1 to 20 wt. %. Water has a great influence on the stability of the catalyst, and the high water content is advantageous to the stability of the catalyst. Therefore, the stability of the catalyst in the anhydrous system of the methyl acetate carbonylation process is a primary problem that should be overcome. In order to solve the problem, a promoter or a co-catalyst such as alkali metal, phosphonium salt, ammonium salt and transition metal catalysts can be added to promote the stability and activity of the catalytic system. In addition, in the methyl acetate carbonylation process for producing acetic anhydride, a small amount of hydrogen must be added in the carbon monoxide feed gas; the presence of hydrogen can reduce the trivalent rhodium [Rh(CO)2I4]− in the catalytic system to a univalent rhodium [Rh(CO)2I2]− having the activity, so as to maintain the activity of the rhodium catalyst. However, an overhigh hydrogen concentration will increase the production of the by-products of vinyl acetate, diacetate and acetone. Currently, all of the researches on methyl acetate carbonylation process mainly focus on the maintenance of the catalyst stability under an anhydrous system, the decrease of corrosiveness to equipments and the increase of the catalytic efficiency. In numerous catalyst researches, the Group VIIIB metals are mostly preferentially chosen for the active metal, and the metal catalysts such as rhodium, iridium, ruthenium, cobalt, nickel etc. have been much studied, of which the rhodium catalyst [U.S. Pat. No. 4,430,273, U.S. Pat. No. 4,333,884 and U.S. Pat. No. 5,298,586] and the nickel catalyst [U.S. Pat. No. 4,002,678, U.S. Pat. No. 4,906,415, U.S. Pat. No. 4,335,059 and U.S. Pat. No. 4,544,511] are most widely studied. The selectivity of them both can be higher than 95%, but the activity of rhodium is ten times or above that of nickel. Therefore, the rhodium catalytic system is mainly used in the current industrial processes.
The addition of one or more promoters into the catalytic system to improve and increase the catalytic efficiency of the catalyst is the most important subject in these researches. U.S. Pat. No. 4,002,678 discloses that under an anhydrous condition, a carbonylation reaction is carried out by using nickel and chromium as the catalyst and carbon monoxide and methyl acetate or dimethyl ether as the raw materials in the presence of a halide and a trivalent organo-nitrogen compound or a trivalent organo-phosphorus compound. EP0391680A1 discloses a process for preparing a carboxylic acid by using an alcohol or its ester under a water-containing condition, in which a quaternary ammonium iodide is used as a stabiliser of the rhodium catalyst. U.S. Pat. No. 4,115,444 discloses a process for preparing acetic anhydride, in which a Group VIIIB noble metal is used as the catalyst, together with multiple promoters comprising at least one metal of Groups IVB, VB, and VIB or a non-noble metal of Group VIIIB, or their compounds and a trivalent organo-nitrogen compound or a trivalent organo-phosphorus compound; the catalyst thereof is rhodium and iridium, the metal promoter is iron, cobalt, nickel, chromium, etc., and the organo-nitrogen compound promoter includes an amine, an imidazole, an imide, an amide, an oxime, etc. CN 1876239A and CN 1778468A both disclose a catalytic system for the synthesis of the carbonyl group of methyl acetate to an acid anhydride by using a rhodium compound as the catalyst and different contents of alkyl iodides, hetero-polyacid salts and alkali metal iodine salts as the promoter; the performance of this catalytic system is improved by the synergistic effect of the hetero-polyacid salts and the catalyst. In the carbonylation process of Taiwan Application No. 97100527, different nitrogen-containing heterocyclic organic promoters are used to form with the rhodium catalyst a stabilized complex, which has the effect of promoting the carbonylation reaction rate; the addition of these organic promoters can lower the reaction temperature or decrease the amount of lithium iodide to be added but maintain the original reaction rate, which has the effect of saving energy and reducing production cost.
Ionic liquids are currently the newest research subject. Due to their characteristic of low vapor pressure, ionic liquids are easily separated and recovered in a catalytic system [Catal. Today, 2002, 74, 157-189] and have good thermal stability, chemical stability, ionic conductivity and polarization potential and thus can be used as an environment-friendly solvent. Therefore, ionic liquids are paid more and more attention to [Chem. Rev, 2004, 248, 2459-2477]. Ionic liquids can play various roles in a catalytic reaction as an organic catalyst, a co-catalyst, the source of a ligand, or the solvent for the reaction [Coordination Chemistry Review 248 (2004) 2459-2477] with certain effects. U.S. Pat. No. 3,689,533 discloses a process for the preparation of acetic acid by the gas-solid phase methanol carbonylation process in the presence of a rhodium-supported catalyst and a halide promoter. However, because the thermal conductivity of gas is extremely smaller than that of liquid, the major problem of the gas phase carbonylation process conducted by using a solid catalyst lies in the removal of reaction heat [U.S. Pat. No. 5,488,143, U.S. Pat. No. 3,717,670 and U.S. Pat. No. 3,689,533]. U.S. Pat. No. 6,916,951 B2 uses a non-volatile ionic liquid as the solvent with a rhodium catalyst dissolved therein to solve the problem of the heat removal in a heterogeneous phase carbonylation process by the participation of the ionic liquid.
There exists so close relation between the organic cation and anion of an ionic liquid that the designing of the structure of ionic liquids greatly influence the process of electron transport and the physical properties of ionic liquids. White et al. [Journal of Molecular Catalysis A: Chemical 238 (2005) 163-174] further verifies that the structure of ionic liquids in the carbonylation reaction will affect the solubility of CO and promote the catalytic performance of active species, and will further affect the stability of the whole reaction system. U.S. Pat. No. 5,298,586 and U.S. Pat. No. 4,430,273 both clearly disclose that the addition of quaternary nitrogen-containing ionic iodides in the rhodium-catalyzed carbonylation process under anhydrous conditions to produce carboxylic acid anhydrides can effectively improve the stability and solubility of rhodium catalysts. In addition to keeping the stability of rhodium catalysts, the promotion of catalytic efficiency is also a goal to be sought. Therefore, developing a process of producing acetic anhydrides which can effectively stabilize rhodium catalysts and maintain a high reaction rate under severe carbonylation conditions is still a main research subject in the future.